The present invention relates to oil and gas well cements and to a novel additive for imparting thixotropic properties thereto. By a thixotropic cement is meant a cement slurry which will develop a low viscosity in turbulent flow, but which exhibits a high viscosity on setting, the transition being reversible.
The use of a thixotropic cement in connection with well treatments is known in the prior art. A cement of this type is disclosed, for example, in U.S. Pat. No. 2,890,752, in which a nonaqueous cement slurry is formed by admixing with portland cement a liquid hydrocarbon base, such as kerosene or diesel oil, with a sufficient amount of an alkali metal soap to produce a thixotropic gel. However, compositions of this character were designed to control the action of the water present in the well on the cement, rather than the qualities of the cement slurry itself.
Accordingly, a need has existed for a cement slurry which will be pumpable as a thin or low viscosity body, but which when the slurry becomes quiescent attains a relatively high viscosity, but without any setting of the cement. A cement having this quality can be used as a squeeze cement for cementing a casing in an oil or gas well, or for displacing drilling mud from a well, and the like, without danger of premature total setting of the cement content if the pumping operation is interrupted. Resumption of pumping operations breaks the thixotropic gel formed and causes the viscosity to diminish to the lower values which prevailed during the initial thin consistency pumping period.